What is called diesel combustion, in which fuel is directly injected into compressed air in the combustion chamber, self-ignites, and is burned by diffusion combustion, has a higher thermal efficiency as compared to combustion by spark ignition. In recent years, in order to enjoy this advantage of diesel combustion also in gasoline engines, technology for causing gasoline to self-ignite and burn by diffusion combustion has been developed.
For example, in the technology disclosed in PTL 1, first fuel injection is performed by an in-cylinder injection valve during the period in the first half of the compression stroke to produce substantially homogenous air-fuel mixture in the entirety of the combustion chamber. Then, the air-fuel mixture produced by the first fuel injection is spark-ignited. Thereafter, second fuel injection is performed to burn the injected fuel. With this combustion, the temperature and pressure in the combustion chamber rise to cause the remaining fuel to self-ignite.
PTL 2 discloses a technology that causes diffusion combustion to occur in a relatively high load operation range of the gasoline engine in which knocking is likely to occur. In the technology disclosed in PTL 2, in the operation range in which knocking is likely to occur, first fuel injection is performed in the cylinder at a time before the top dead center of the compression stroke to burn the injected fuel by spark ignition. Then, second fuel injection is performed in the cylinder at a time after the top dead center of the compression stroke, at which the pressure of in the combustion chamber has been raised by the combustion, to cause the injected fuel to burn by diffusion combustion. In this technology, either homogenous fuel injection that produces homogeneous air-fuel mixture in the entirety of the combustion chamber or stratified fuel injection that produces air-fuel mixture only in a part of the interior space of the combustion chamber is selectively performed as the first fuel injection, depending on the engine speed.
PTL 3 discloses a technology for causing diesel combustion using as fuel natural gas having a relatively high self-ignition temperature. In the technology disclosed in PTL 3, fuel injection is performed in a specific spark ignition region in the combustion chamber at a time in the early or middle stage of the compression stroke to produce air-fuel mixture that can be spark-ignited. The air fuel mixture produced in this spark ignition region is ignited at a time immediately before the top dead center of the compression stroke to cause spark-ignition combustion. Thus, a high-temperature, high-pressure condition enabling self-ignition of natural gas is established in the combustion chamber. Thereafter, fuel is injected directly into the combustion chamber in the high-temperature, high-pressure condition to cause the fuel to burn by diesel combustion.